PHY2028 Troubleshooting Op-Amp Circuits

Introduction

The ability to diagnose and cure problems in a systematic manner is an exceptionally valuable skill. In PHY2028 students are given a heap of components, a breadboard, and measuring equipment. This virtually guarantees that nothing will work first time, and students are forced to develop troubleshooting skills.

Students: please ensure that you can answer 'yes' to the relevant questions below before asking a demonstrator for help.

The Method

Carry out quick and easy checks first: start with a visual inspection, then use a multimeter, finally an oscilloscope.

Before You Start

1. Is the circuit diagram open on the bench?
2. Do you understand how it is supposed to work?
3. Are you sure of the pin-outs and polarity of each device?

Power and Grounds

1. Is the power supply is working?
   Check for stable outputs with a digital multimeter (DMM)
2. Are op-amps connected to both supply rails?
   Check voltages with respect to ground at pins of each device. Do not cause short-circuits with the DMM probes
3. Is the ground rail continuous?
   Check voltage between power supply rail and points on circuit that should be at ground.

Passive Components

1. Are the resistor and capacitor values correct?
   Check them with a DMM/capacitance meter. If you are not confident that you can estimate the the effects of other components for in situ measurements, carefully disconnect one end of the component from from the board. Don't simultaneously touch both probes with your hands when making the measurement or you will get misleading results.
2. Are potentiometers being used correctly?
   Check that the wiper voltage varies in a reasonable manner as the potentiometer is adjusted. Avoid using potentiometers as variable resistors.
3. Are switches of the correct type?
   Have you checked your assumptions about which terminal is which with a DMM?

Op-Amps

If you suspect an op-amp is faulty, check it by substitution but first:

1. Is each pin of the op-amp connected?
   Check visually that none of its pins have become wrapped under its body instead of being inserted into the board.
2. Is the output finite?
   If the output is within a volt or so of either power rail then either it has failed, or there is an excessive voltage at its inputs.
3. Is the input consistent with the output?
   Measure the voltage between the inverting and non-inverting inputs, it should be within millivolts of zero.

Offset Voltages

A few millivolts of offset at the input of a system with a high DC gain can be amplified to the point that it saturates the final stage. Many op-amps have offset adjust facilities which can reduce the offset by something like a factor of 10. Offsets are temperature dependent.

Instability

Instability typically appears as high-frequency 'fuzz' on the output signal oscilloscope trace. Breadboard circuits are very prone to it because of inter-track capacitance and long components leads. Try the following:

1. Organise the physical layout of the circuit to keep the input and output stages separate.
2. Decouple the breadboard power supply rails with a circa 0.1µF capacitors to ground.
3. Work out the loop-gain (see worksheet 1) and study the op-amp manufacturer's data-sheet.
4. Don't use op-amps with an unnecessarily good frequency response, e.g. use a LM741 instead of an LF411 where possible.

Divide and Conquer

If you have a complex system involving several stages:

1. Divide the system into sub-circuits that can be tested individually.
   If you want to do an open-loop test on a closed-loop system use a signal generator or voltage source to inject a simulation of the closed-loop signal at the point where you open the loop.
2. Check the signal at the input and output of each section, using an oscilloscope if appropriate.
   It is possible for a faulty section to load its predecessor in the chain.

How to Kill a Working Circuit

1. Short-circuit a supply rail to something sensitive with the DMM probes when checking the PSU or by dropping something (e.g. a screwdriver) onto the working circuit.
2. Apply power for an instant to only one rail of a circuit that requires two rails.
3. Make changes to the circuit without switching off the PSU first.